Preheat your oven to 180°C (350°F).

Roll out the shortcrust pastry and line a tart pan with it. Prick the bottom with a fork to prevent it from puffing up during baking.

Place a sheet of parchment paper over the pastry and fill it with baking beans or rice to weigh it down. Bake for 15 minutes, then remove the weights and parchment and bake for an additional 10 minutes until lightly golden. Remove from the oven and let it cool.

In a saucepan, heat the milk over medium heat until it is just about to boil. Remove from heat.

In a mixing bowl, whisk together the eggs, sugar, cornstarch, vanilla extract, lemon zest, and a pinch of salt until well combined.

Gradually pour the hot milk into the egg mixture, whisking continuously to temper the eggs and prevent them from cooking.

Pour the mixture back into the saucepan and return to medium heat. Cook while stirring constantly until the custard thickens and begins to bubble. This should take about 5-7 minutes.

Once thickened, pour the custard into the cooled tart shell, smoothing the top with a spatula.

Bake the tart in the oven for 25-30 minutes until the top is set and lightly golden.

Remove from the oven and allow to cool to room temperature. Refrigerate for at least 2 hours before serving to allow the custard to fully set.

Serve chilled, dusted with powdered sugar if desired.